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Abstract

Accretion in meandering rivers does not exclusively occur in point bars. This is demonstrated by the River Tywi, a meandering gravel-bed stream in South Wales, in which deposition occurs at the concave sides of meander bends. These deposits, termed counterpoint bars in this paper, form at sites commonly assumed to be ones of erosion. There are two types of counterpoint bar in the Tywi; fine-grained examples, similar to the concave bank benches of Page and Nanson (1982), and large gravel bars. Gravel counterpoint bars are a little known type. They are bulbous to elongate in plan view, are as much as 30-40 m wide and 60-70 m long. They consist of a bar platform, secondary channel and riffle.

Gravel counterpoint bars appear to have a relatively high preservation potential in the Tywi valley. This is suggested by historical maps and the presence of several abandoned examples on the floodplain. These form elongate and bulbous assemblages of two or more bars. Morphology and sedimentary structures are distinct from the more familiar fine-grained examples described from other rivers; however, gravel counterpoint bars may be more difficult to distinguish from gravel point bars if preserved in the geologic record.

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